Assay of denatured lipoproteins

Abstract: Disclosed herein is an assay of a denatured lipoprotein, in which the denatured site of the denatured lipoprotein contained in a vital sample is exposed to the surface of its lipoprotein particle upon the reaction of an antibody, which recognizes the denatured lipoprotein, with the vital sample containing the denatured lipoprotein.

Description: BACKGROUND OF THE INVENTION Field of the Invention: The present invention relates to a reliable assay of denatured lipoproteins which are considered to involve arteriosclerosis and present in the living body. Description of the Background Art: Arteriosclerosis is known to be the main cause of ischemic heart diseases, cerebral infarction and the like. The mechanism causative of arteriosclerosis is reported to form atheroma due to accumulation of smooth muscle cells, connective tissues and a great amount of lipids (mainly, cholesterol esters) on a vascular subendothelium, resulting in the hypertrophy of the arterial wall and advanced induration, which finally brings about the hypofunction of the artery. In particular, it is considered that the atheroma recognized in the initial lesion of this disease is formed from foam cells produced by the accumulation of a denatured low density lipoprotein (LDL) into macrophages. However, the true character of the denatured LDL which transforms the macrophages into the foam cells in the living body is unknown to date. However, it has recently been explicated that there is a possibility that two kinds of denatured LDLs one of which is a denatured LDL formed by the interaction between a vascular endothelial cell which is reported to play an important role in arteriosclerosis and LDL [Quinn, M. T., Parthasarathy, S., Fong, L. G. & Steingerg, D., Proc. Natl. Acad. Sci. USA, 84 , 2995-2998 (1987)], and the other of which is an LDL modified with malondialdehyde (MDA) known as an end metabolic product of lipid peroxides and thromboxane A 2 , which also play an important role in arteriosclerosis [Fogelman, A. M., Shechter, I., Seager, J., Hokom, M., Child, J. S. & Bdwards, P. A., Proc. Natl. Acad. Sci. USA, 77 , 2214-2218 (1980)], may deeply participate in the initial lesion of arteriosclerosis. An assay of these denatured LDLs in the living body has hence been regarded as important. Some methods have heretofore been reported as methods for assaying denatured LDLs. For example, Gonen et al., and Salmon et al. apply an EIA competition analysis in which MDA-modified LDL coated on the plate, and an antibody which recognizes the MDA-modified LDL are used [Gonen, B., Fallon, J. J. & Baker, S. A, Atherosclerosis, 65 , 265-272 (1987)] and an EIA sandwich technique in which an immobilized antibody which recognizes MDA-modified LDL and an enzyme-labeled antibody which recognizes LDL are used [Salmon, S., Maziere, C., Theron, L., Beucler, I., Ayrault-Jarrier, M., Goldstein, S. & Polonovski, J., Biochim. Biophys. Acta, 920 , 215-220 (1987)], respectively, to assay extracts from a focus of arteriosclerosis or the MDA-modified LDL in the blood However, Gonen et al. have failed to detect the MDA-modified LDL in the arteriosclerosis tissue according to this analysis. On the other hand, Salmon et al. have reported that although the assay of the MDA-modified LDL in the blood was performed as to 65 specimens, only 14 specimens were able to be detected, but the residual 51 specimens were unable to be detected. As described above, it has heretofore been known to assay MDA-modified LDL artificially synthesized. It has however been impossible to effectively assay MDA-modified LDL, i.e., a denatured LDL, in vital samples. On the other hand, it has been known that lipoprotein (a) [Lp(a)] involves the causation of the coronary artery disease independent of other risk factors in the causation of arteriosclerotic diseases. In recent years, attention has been paid to the participation of a denatured Lp(a) as a mechanism causative of this disease [Loscalzo. J., Arteriosclerosis, 10 , 672-679 (1990)], that is to say, a mechanism that the denatured Lp(a) accumulates on the coronary artery, and macrophages attracted thereto incorporate it therein, whereby they are transformed into foam cells. As recent reports supporting this hypothesis, may be mentioned a report that when Lp(a) is treated with copper or modified with malondialdehyde (MDA) to be converted to a denatured Lp(a) having a minus charge, its engorgement into macrophages increases [Haberland, M. E., Fless, G. M., Scanu, A. M. & Fogelman, A. M., J. Biol. Chem., 267 , 4143-4151 (1992)], and a report that Lp(a) accumulates on a focus of arteriosclerosis, and this Lp(a) has a more negative charge compared with native (normal) Lp(a) as determined by agarose electrophoresis [O'Neil, J. A., Pepin, J. M., Smejkal, G., et al, Arteriosclerosis, 10 , 812a (1990)]. As described above, it has been explicated that denatured Lp(a)s have a possibility of deeply participating in the causation of arteriosclerosis, and an assay of the denatured Lp(a)s in the living body has hence been regarded as important. A great number of methods have heretofore been reported as methods for assaying denatured Lp(a)s in the living body. For example, Fless et al. have established an EIA sandwich technique making good use of a likeness of Lp(a) to a product obtained by binding LDL with apo(a), in which an antibody which recognizes apoB, i.e., a main structural protein, and another antibody which recognizes apo(a) are used [Fless, G. M., Snyder, M. L. & Scanu, A. M., Lipid Res., 30 , 651-662 (1989)]. As described above, it has heretofore been known to assay Lp(a). However, these methods are not intended to specifically detect denatured Lp(a)s in biological samples, which are considered to have a possibility of more deeply participating in the causation of arteriosclerosis. On the other hand, among various kinds of lipoproteins, those in which the existence of denatured lipoproteins has been recognized include high density lipoprotein (HDL) and the like. However, no assay has been yet developed even on these denatured lipoproteins [Morel, D. W., Biochem. Biophys. Res. Commun., 200 , 408-416 (1994); Maziere, J. C., Myara, I., Santus, R. & Mazier E. C., Atherosclerosis, 104 , 213-219 (1993)]. SUMMARY OF THE INVENTION Therefore, it is an object of the present invention to provide a method of selectively assaying denatured lipoproteins in biological samples. In view of the foregoing circumstances, the present inventor has carried out an extensive investigation. As a result, it has been found that the fact that even when an antibody (hereinafter referred to as "anti-denatured lipoprotein antibody") which recognizes a denatured lipoprotein is used, the denatured lipoprotein in the living body can not be assayed is attributed to the fact that the denatured site of the denatured lipoprotein is hidden in the interior of its lipoprotein particle, whereby the denatured lipoprotein is inactivated, and hence that if this denatured site is exposed to the surface of the lipoprotein particle, the denatured lipoprotein can react with the anti-denatured lipoprotein antibody to selectively assay it, thus leading to completion of the present invention. In an aspect of the present invention, there-is thus provided an assay of a denatured lipoprotein, which comprises exposing the denatured site of the denatured lipoprotein contained in a biological sample to the surface of its lipoprotein particle and reacting an anti-denatured lipoprotein antibody with the biological sample containing the denatured lipoprotein. According to the present invention, the denatured site of a denatured lipoprotein, which has been hidden in the interior of its lipoprotein particle, can be exposed to the surface of the lipoprotein particle to succeed in reaction with the anti-denatured lipoprotein antibody, whereby the denatured lipoprotein in the living body can be reliably assayed. BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings, in which: FIG. 1 diagrammatically illustrates the effect of sodium dodecyl sulfate (SDS) on an assaying system for MDA-modifed LDL in a human serum using an anti-human MDA-modified LDL monoclonal antibody; FIG. 2 diagrammatically illustrates the relationship between the concentration of MDA-modified HSA or HSA and inhibitory effect upon the reaction of an anti-human MDA-modified LDL monoclonal antibody with a mixture of a human serum and MDA-modified HSA or HSA in the presence of SDS; FIG. 3 diagrammatically illustrates reactivities of an anti-human MDA-modified LDL monoclonal antibody to MDA-modified LDL, LDL, VLDL (very low density lipoprotein), IDL (intermediate density lipoprotein) and HDL, respectively; FIG. 4 diagrammatically illustrates the relationship between the dilution of a human serum and enzymatic activity upon assaying MDA-modified LDL in a human serum in the absence or presence of sodium dodecylbenzenesulfonate using an anti-human MDA-modified LDL monoclonal antibody; FIG. 5 diagrammatically illustrates the relationship between individual sera and enzymatic activity upon assaying MDA-modified LDL in sera of persons of health, diabetics and cases of an arteriosclerotic disease according to the method of the present invention; FIG. 6 diagrammatically illustrates the effect of SDS on an assaying system for MDA-modified Lp(a) in a human serum using an anti-MDA-bound Lp(a) monoclonal antibody; FIG. 7 diagrammatically illustrates the relationship between the concentration of MDA-modified HSA or HSA and inhibitory effect upon the reaction of an anti-MDA-modified Lp(a) monoclonal antibody with a mixture of a human serum and MDA-modified HSA or HSA in the presence of SDS; FIG. 8 diagrammatically illustrates the relationship between the dilution of a human serum and enzymatic activity upon assaying MDA-modified Lp(a) in a human serum in the absence or presence of sodium dodecylbenzenesulfonate using an anti-MDA-modified Lp(a) monoclonal antibody; FIG. 9 diagrammatically illustrates the relationship between the dilution of a human serum and enzymatic activity upon assaying MDA-modified Lp(a) in a human serum in the absence or presence of cetyltrimethylammonium chloride using an anti-MDA-modified Lp(a) monoclonal antibody; FIG. 10 diagrammatically illustrates the relationship between the dilution of a human serum and enzymatic activity upon assaying MDA-modified Lp(a) in a human serum in the absence or presence of a polyoxyethylene secondary alkyl ether using an anti-MDA-modified Lp(a) monoclonal antibody; and FIG. 11 diagrammatically illustrates the relationship between individual sera and enzymatic activity upon assaying MDA-modified Lp(a) in sera of persons of health, diabetics and cases of an arteriosclerotic disease according to the method of the present invention. DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS Denatured lipoproteins intended to be assayed in the present invention include denatured LDL, denatured Lp(a), denatured HDL and the like. As described above, the denatured LDL deeply participates in the causation of arteriosclerosis. Besides, the denatured Lp(a) deeply participates in the causation of sclerosis particularly in a coronary artery among arteries. Further, the denatured HDL also deeply participates in the causation of arteriosclerosis. No particular limitation is imposed on the biological sample used in the present invention so far as it is a sample in which a denatured lipoprotein may exist. However, samples thereof include blood, plasma, serum and the like. Of these, plasma or serum is particularly preferred. The anti-denatured lipoprotein antibody used in the present invention may be either a polyclonal antibody or a monoclonal antibody so far as it reacts with the denatured lipoprotein, but does not react with its normal lipoprotein. However, the monoclonal antibody is preferred. Examples of the anti-denatured lipoprotein antibody include anti-MDA-modified lipoprotein antibodies and anti-hapten antibodies which recognize an MDA residue. Of these, anti-MDA-modified lipoprotein monoclonal antibodies are particularly preferred. Specific examples thereof include an anti-MDA-modified LDL antibody, an anti-MDA-modified Lp(a) antibody and an anti-MDA-modified HDL antibody. In the case of Lp(a), the anti-MDA-modified LDL antibody may be used because it has almost similar composition of LDL binding with apo(a). The monoclonal antibody can be produced in the following manner in accordance with, for example, the method described in Japanese patent Application Laid-Open No. 173096/1992. A purified lipoprotein is first prepared from a fresh human serum by an ultracentrifugation known per se in the art. MDA is then reacted with this lipoprotein to prepare an MDA-modified lipoprotein. The MDA-binding reaction of the lipoprotein is preferably performed for 30 minutes to 24 hours usually at a pH of 6-7 and a reaction temperature of 20-40°C. Since this reaction can be stopped by cooling the reaction mixture to 4°C, the reaction mixture is cooled to 4°C to stop the reaction. The reaction mixture thus cooled is dialyzed at 4°C to a buffer solution kept at pH 6-8 and added with 10-100 µM of EDTA as needed. This human MDA-modified lipoprotein can be used as an immunogen to prepare an anti-human MDA-modified lipoprotein monoclonal antibody by an already-known cell fusion means. More specifically, a mouse is immunized to the human MDA-modified lipoprotein, and a spleen is taken out of the mouse after a predetermined period of time. After the cell of the spleen taken out is fused with a myeloma cell in the presence of polyethylene glycol, the fused cell is cultured for a predetermined period of time, whereby a hybridoma strain which produces an anti-human MDA-modified lipoprotein monoclonal antibody high in specificity can be obtained owing to a difference in reactivity to the lipoprotein and the MDA-modified lipoprotein among antibodies produced in a culture supernatant. The thus-obtained hybridoma cell strain is cultured in vitro or in vivo to obtain an anti-MDA-modified lipoprotein antibody. As a means for exposing the denatured site of the denatured lipoprotein in a biological sample to the surface of its lipoprotein particle, which is most important in the present invention, may be mentioned a surfactant treatment. More specifically, when a surfactant is caused to act on the denatured lipoprotein in the living body, the lipoprotein undergoes a structural change, whereby the denatured site hidden in the interior of the lipoprotein particle is exposed to the surface of the lipoprotein particle. In the method of the present invention, the surfactant is caused to coexist in a reaction system containing the biological sample and the anti-denatured lipoprotein antibody. As a result, a reaction of the antibody with the denatured lipoprotein takes place, whereby the denatured lipoprotein in the vital sample can be selectively assayed. Examples of the surfactant used include anionic surfactants, cationic surfactants, amphoteric surfactants and nonionic surfactants. Of these, the anionic surfactants are preferred. For example, mono- or di-C 8-18 alkyl quaternary ammonium salts, C 8-18 alkylsulfates, C 8-18 alkylbenzenesulfonates, C 8-18 alkylphosphates, C 8-18 alkyl ether sulfates, C 8-18 alkyl ether benzenesulfonates, polyoxyethylene secondary alkyl ethers and the like may be mentioned. Of these, cetyltrimethylammonium chloride, benzalkonium chloride, sodium dodecyl sulfate (SDS), sodium dodecylbenzenesulfonate, diethanolamine alkylphosphates, polyoxyethylene secondary alkyl ethers (added with 9 mols of ethylene oxide) are particularly preferred. The surfactant may preferably be used in a concentration ranging from 0.1 to 50 mM, particularly from 0.5 to 20 mM. In particular, SDS may preferably be used in a range of 1-5 mM. No particular limitation is imposed on the means for assaying the denatured lipoprotein the denatured site of which has been exposed to the surface of its lipoprotein particle so far as it is an immunoassay known per se in the art. Examples of methods usable in the present invention include immunoturbidimetry, nephelometric immunoassay, latex agglutination method, radioimmunoassay, enzyme immunoassay. In particular, enzyme immunoassay is preferred because it is excellent in specificity, sensitivity and utility. The method of the present invention will hereinafter be described in detail taking the case of the enzyme immunoassay. It has been suggested that MDA-modified proteins other than human MDA-modified lipoproteins exist in human sera [Kergonou, J. F., Bruna, B., Pennacino, I. & Ducousso, R., Advances in the Biosciences, 71 , 121-124 (1988)]. Therefore, it is first necessary to take account of the possibility that the anti-human MDA-modified lipoprotein antibody may react with the other MDA-modified proteins. Accordingly, in the case where the denatured LDL is assayed, it is preferable to use a sandwich immunoassay making use of both antibodies which respectively recognize human MDA-modified LDL and human apoB constituting LDL so as to enhance the specificity for the detection of the human MDA-modified LDL. More specifically, one of an anti-human MDA-modified LDL monoclonal antibody and an anti-human apoB antibody is immobilized on an insoluble carrier, and the other is labeled with a labeling substance. These antibodies are then brought into contact with a biological sample to perform the sandwich immunoassay, thereby determining the human MDA-modified LDL. In the case where the denatured Lp(a) is assayed, it is preferable to use a sandwich immunoassay making use of an antibody which recognizes human apo(a) constituting human MDA-modified Lp(a) so as to enhance the specificity for the detection of the human MDA-modified Lp(a). More specifically, one of an anti-human MDA-modified Lp(a) monoclonal antibody and an anti-human apo(a) antibody is immobilized on an insoluble carrier, and the other is labeled with a labeling substance. These antibodies are then brought into contact with a biological sample to perform the sandwich immunoassay, thereby determining the human MDA-modified Lp(a). Further, in the case where the denatured HDL is assayed, it is preferable to use a sandwich immunoassay making use of an antibody which recognize human apoA-I or apoA-II constituting human MDA-modified HDL so as to enhance the specificity for the detection of the human MDA-modified HDL. More specifically, one of an anti-human MDA-modified HDL monoclonal antibody and an anti-human apoA-I antibody or human apoA-II antibody is immobilized on an insoluble carrier, and the other is labeled with a labeling substance. These antibodies are then brought into contact with a biological sample to perform the sandwich immunoassay, thereby determining the human MDA-modified HDL. Examples of the insoluble carrier used herein include various kinds of synthetic polymers such as polyethylene and polypropylene, glass, silicon, insoluble polysaccharides, and the like. These carriers may be used in the form of a sphere, a rod, fine particles or the like, or as a test tube, a microtiter plate or the like. The preparation of the insolubilized antibody is conducted by binding an antibody to an insoluble carrier by physical adsorption or covalent binding. Examples of the labeling substance used in the labeled antibody include radioisotopes, enzymes, various kinds of fluorescent materials and fine particles such as colloidal gold. For example, in the case where an enzyme is used as a labeling substance for the labeled antibody, the enzyme-labeled antibody can be prepared by any known method. As necessary, an antibody to be used may be partially degested into F(ab') 2 by a suitable protease or further into Fab' in the presence of a reducing agent, and such a fragment may be then labeled with an enzyme. Examples of the enzyme used in labeling the antibody include β-D-galactosidase, peroxydase, alkaline phosphatase, glucose oxidase and the like. The immune reaction may be either a one-step reaction or a two-step reaction. If the reaction is conducted by the one-step reaction, the surfactant may be added during the immune reaction. If the immune reaction is performed by two steps, the biological sample is first brought into contact with the insolubilized antibody in the first reaction to bind an antigen into an insolubilized antibody-antigen complex. In the second reaction, the enzyme-labeled antibody is then bound to this complex into an insolubilized antibody-antigen-enzyme-labeled antibody complex. In this two-step reaction, an buffer solution with which the surfactant has been caused to coexist is used in a liquid reaction mixture in the stage that the anti-denatured lipoprotein antibody is brought into contact with the antigen. More specifically, when the anti-denatured lipoprotein antibody is used as the insolubilized antibody or the labeled antibody, the buffer solution containing the surfactant therein is caused to coexist with the insolubilized antibody in the first reaction or the labeled antibody in the second reaction to conduct the immune reaction. As the buffer solution, may be used those having a pH ranging from 6 to 10. As a buffer, glycine, tris, phosphate, imidazole, triethanolamine, barbiturate or Good's buffer may preferably be used in a range of 10-100 mM. In particular, it is preferable to use a buffer of glycine, HEPES (a kind of Good's buffer) or phosphate in a concentration of 10-50 mM so as to keep pH 7-9. In order to stabilize the antigen and the antibodies in the liquid reaction mixture, it is further desirable that a suitable protein be caused to coexist with them. Examples of the protein include BSA, HSA, gelatin and the like. The protein may preferably be used in a concentration ranging from 0.05 to 2.0% (w/v). It is particularly preferable to use BSA in a range of 0.1-1.0% (w/v). As necessary, a salt such as NaCl or KCl may be added in a concentration ranging from 10 to 300 mM for controlling ionic strength, and besides an antiseptic such as chlorhexidine or NaN 3 may be added for preventing the growth of bacteria. In the case where the immune reaction is performed in two steps, no particular limitation is imposed on the kind of a buffer used in a step that the other anti-apoB antibody, anti-apo(a) antibody, anti-apoA-I antibody or anti-apoA-II antibody is reacted with the antigen, even when such an antibody is used in either the insolubilized antibody or the labeled antibody so far as the buffer can keep the immune reaction a suitable pH of 6-9. The label activity of the complex thus obtained is then measured, whereby the amount of the denatured lipoprotein in the biological sample can be determined. The present invention will hereinafter be described more specifically by reference to the following examples. Incidentally, an anti-human MDA-modified LDL monoclonal antibody No. 29225 to be used in the following examples was produced in accordance with the procedure described in Example 1 of Japanese Patent Application Laid-Open No. 173096/1992. Example 1: Effect of SDS on assaying system of MDA-modified LDL The anti-human MDA-modified LDL monoclonal antibody No. 29225 was first prepared with a phosphate buffer solution (pH: 7.2)-saline (hereinafter referred to as "PBS") so as to give a concentration of 16 µg/ml. The thus-obtained preparation was then poured in portions of 100 µl/well into microplates. Each of the microplates was left at rest overnight at 4°C to immobilize the antibody. After the microplate was then fully washed with distilled water, the antibody was blocked overnight at 4°C in PBS containing 4% of non-fat dry milk. The antibody-bound plates thus obtained were used in reaction. The following assaying procedure was followed to conduct the reaction. Namely, 25 mM of glycine buffer solution (pH: 9.0), which contained 0.2% of bovine serum albumin (BSA), 75 mM of NaCl and 0.1% of NaN 3 , and were added separately with SDS in concentrations of 0, 0.75, 2.25, 3.75 and 5.25 mM (hereinafter called "SDS-containing BSA-Gly"), were separately added in portions of 100 µl/well to the antibody-bound plates. Dilute samples of a serum of person of health, which had been obtained by diluting the serum from 1/200 to 1/3200 in five steps with BSA-Gly, were separately poured in portions of 50 µl/well into the antibody-bound plates to start reaction. After reacted for 1 hour at 37°C, the plates were fully washed. A β-galactosidase-labeled anti-human apoB monoclonal antibody solution was then poured in portions of 100 µm/well into the antibody-bound plates to conduct reaction for 1 hour at 37°C. After fully washing the plates, O-nitrophenyl-β-D-galactopyranoside was used as a substrate to perform an enzyme reaction at 37°C. After 1 hour, a 0.1 M Na 2 CO 3 solution was added in portions of 100 µm/well to the plates to stop the reaction. Thereafter, the absorbances of the liquid reaction mixtures in the individual plates were measured at a wavelength of 414 nm. The results are as illustrated in FIG. 1, and revealed that in the reaction system to which no SDS is added, the MDA-modified LDL in the serum cannot be detected at all, while the MDA-modified LDL in the serum can be determined by adding SDS to the reaction system. Example 2: Identification of specificity of assaying system of MDA-modified LDL making use of MDA-modified human serum albumin (HSA) The preparation of MDA modified HSA was conducted in accordance with the method described in Example 1 of Japanese Patent Application Laid-Open No. 173096/1992. More specifically, HSA and MDA were dissolved in 50 mM of a phosphate buffer solution (pH: 6) so as to give final concentrations of 3 mg/ml and 16.7 mM, respectively, to react them at 25°C for 5 hours. The liquid reaction mixture was cooled to 4°C to stop the reaction, and then dialyzed at 4°C to PBS containing 100 µM of EDTA. The thus-obtained product was used as MDA-modified HSA. Antibody-bound plates prepared in the same manner as in Example 1 were used in reaction. Samples used in the reaction were prepared by mixing, in a proportion of 1:1, a dilute serum of person of health which had been obtained by diluting the serum to 1/3200 with 25 mM of a phosphate buffer solution (pH: 7.2) containing 0.5% of BSA, 150 mM of NaCl and 0.05% of NaN 3 in advance (hereinafter called "BSA-PBS") with individual BSA-PBSs that the concentrations of MDA-modified HSA or HSA contained therein were 0, 1, 10 and 100 µg/ml. The following assaying procedure was followed. Namely, BSA-PBS containing 2.5 mM of SDS was added in portions of 100 µl/well to the antibody-bound plates. Thereafter, the samples prepared in the above-described manner were separately poured in portions of 50 µl/well into the plates to start reaction. Thereafter, the procedure of Example 1 was followed to conduct the succeeding steps of the assay. This example is intended to use the nature of the MDA-modified HSA that reacts with the anti-MDA-modified LDL monoclonal antibody coated on the plate, so as to demonstrate that if the substance detected in the assay of the present invention is MDA-modified LDL in the blood, the MDA-modified LDL undergoes competitive inhibition against the MDA-modified HSA added to the reaction system. The results are as illustrated in FIG. 2 and revealed that the addition of HSA does not affect the reaction, but the addition of MDA-modified HSA inhibits the reaction in proportion to the concentration thereof. This indicates that the assay of the present invention can specifically detect the MDA-modified LDL in the blood. Example 3: Reactivity of assaying system to lipoprotein fractions Lipoprotein fractions were first obtained from a fresh human serum by an ultracentrifugation known per se in the art. More specifically, after chylomicron was removed from the fresh human serum by centrifugation, KBr was added to adjust the specific density d of the serum. The thus-adjusted serum was fractionated by step-wise ultracentrifugation into fractions respectively having specific densities of d < 1.019, 1.019 < d < 1.063, and 1.063 < d < 1.21, which were regarded as "very low density lipoprotein (VLDL) + intermediate density lipoprotein (IDL)", "LDL" and "high density lipoprotein (HDL)", respectively. The individual lipoprotein fractions were dialyzed to PBS containing 100 µM of EDTA and then subjected to determination of protein by the Lowry method. A part of LDL was bound with MDA (MDA-modified LDL was prepared in the same manner as the MDA modification of HSA in Example 2), and the MDA-modified LDL thus obtained was then dialyzed to PBS containing 100 µM of EDTA like the individual lipoprotein fractions and moreover subjected to determination of protein by the Lowry method. Portions of the thus-prepared lipoproteins were respectively diluted with BSA-Gly (see Example 1) so as to give concentrations of 0.01, 0.04, 0.2 and 1.0 µg/ml for the MDA-modified LDL, and of 0.5, 5.0, 50 and 500 µg/ml for the other lipoprotein fractions, thereby preparing samples. The following assaying procedure was followed. Namely, BSA-Gly containing 3.75 mM of SDS was added in portions of 100 µl/well to antibody-bound plates prepared in the same manner as in Example 1, and the samples prepared above were separately poured in portions of 50 µl/well into the plates to start reaction. Thereafter, the procedure of Example 1 was followed to conduct the succeeding steps of the assay. The results are as illustrated in FIG. 3 and revealed that even LDL and VLDL + IDL, which are lipoproteins containing apoB, show only reactivity as low as about 1/1000 compared with that of the MDA-modified LDL. Example 4: Assay of MDA-modified LDL added with sodium dodecylbenzenesulfonate as anionic surfactant Antibody-bound plates prepared in the same manner as in Example 1 were used in reaction. The following assaying procedure was followed. Namely, PBS (see Example 1) containing 0.1% of BSA, to which 8.6 mM of sodium dodecylbenzenesulfonate had been added, was added in portions of 100 µl/well to the antibody-bound plates. Further, dilute samples of a human serum, which had been obtained by diluting the serum from 1/400 to 1/3200 in four steps with PBS containing 0.1% of BSA, were separately poured in portions of 50 µl/well into the antibody-bound plates to start reaction. Thereafter, the procedure of Example 1 was followed to conduct the succeeding steps of the assay. The results are as illustrated in FIG. 4, and revealed that in the reaction system to which no sodium dodecylbenzenesulfonate is added, the MDA-modified LDL in the serum cannot be detected at all, while the MDA-modified LDL in the serum can be determined by adding sodium dodecylbenzenesulfonate to the reaction system. Example 5: Sera of 49 persons of health, 31 diabetics and 21 cases of an arteriosclerotic disease were used as specimens to determine the amount of MDA-modified LDL contained in each serum according to the method of Example 1. Namely, antibody-bound plates prepared in the same manner as in Example 1 were used in reaction, and the following assaying procedure was followed. Each of the sera diluted to 1/401 and BSA-Gly containing 3.75 mM of SDS were added in proportions of 50 µl/well and 100 µl/well, respectively, to the antibody-bound plate. Thereafter, the procedure of Example 1 was followed to conduct the succeeding steps of the assay. The results are as illustrated in FIGs. 5(1)-5(3), and revealed that the MDA-modified LDL exists in plenty in the sera of the cases of diabetes and arteriosclerotic disease compared with the sera of the persons of health. Example 6: Effect of SDS on assaying system of MDA-modified Lp(a) An anti-human MDA-modified Lp(a) monoclonal antibody No. 29225 (corresponding to the anti-human MDA-modified LDL monoclonal antibody described in Japanese Patent Application Laid-Open No. 173096/1992) was first prepared with PBS so as to give a concentration of 16 µg/ml. The thus-obtained preparation was then poured in portions of 100 µl/well into microplates. Each of the microplates was left at rest overnight at 4°C to immobilize the antibody. After the microplate was then fully washed with PBS containing 0.05% of Tween 20 (hereinafter referred to as "PBST"), the antibody was blocked overnight at 4°C in PBS containing 4% of non-fat dry milk. The antibody-bound plates thus obtained were used in reaction. The following assaying procedure was followed to conduct the reaction. Namely, 25 mM of HEPES buffer solution (pH: 7.0), which contained 0.2% of BSA, 30 mM of NaCl and 0.1% of NaN 3 (hereinafter called "BSA-HEPES"), and were added separately with SDS in concentrations of 0, 3.75, 6.75, 9.75 and 15.75 mM, were separately added in portions of 100 µl/well to the antibody-bound plates. Dilute samples of a human serum, which had been obtained by diluting the serum from 1/50 to 1/400 in four steps with BSA-HEPES, were separately poured in portions of 50 µl/well into the antibody-bound plates to start reaction. After reacted for 1 hour at room temperature, the plates were fully washed. A β-galactosidase-labeled anti-human apo(a) monoclonal antibody solution was then poured in portions of 100 µm/well into the antibody-bound plates to conduct reaction for 1 hour at room temperature. After fully washing the plates, O-nitrophenyl-β-D-galactopyranoside was used as a substrate to perform an enzyme reaction at room temperature. After 1 hour, a 0.1 M Na 2 CO 3 solution was added in portions of 100 µm/well to the plates to stop the reaction. Thereafter, the absorbances of the liquid reaction mixtures in the individual plates were measured at a wavelength of 414 nm (reference wavelength: 690 nm). The results are as illustrated in FIG. 6, and revealed that in the reaction system to which no SDS is added, the MDA-modified Lp(a) in the serum cannot be detected at all, while the MDA-modified Lp(a) in the serum can be determined by adding SDS to the reaction system. Example 7: Identification of specificity of assaying system of MDA-modified Lp(a) making use of MDA-bound human serum albumin (HSA) The preparation of MDA modified HSA was first conducted. HSA and MDA were dissolved in 50 mM of an acetate buffer solution (pH: 4) so as to give final concentrations of 9 mg/ml and 133.3 mM, respectively, to react them at 25°C for 5 hours. The liquid reaction mixture was cooled at 4°C to stop the reaction, and then dialyzed at 4°C to a 25 mM HEPES buffer solution (pH: 7) containing 30 mM of NaCl and 0.1% of NaN 3 . The thus-obtained product was used as MDA-modified HSA. Besides, antibody-bound plates prepared in the same manner as in Example 6 were used in reaction. Samples used in the reaction were prepared by mixing, in a proportion of 1:1, a dilute human serum which had been obtained by diluting the serum to 1/50 with BSA-HEPES (see Example 6) with individual BSA-HEPESs that the concentrations of MDA-bound HSA or HSA contained therein were 0, 10, 100 and 500 µg/ml. The following assaying procedure was followed. Namely, BSA-HEPES containing 6.75 mM of SDS was added in portions of 100 µl/well to the antibody-bound plates. Thereafter, the samples prepared in the above-described manner were separately poured in portions of 50 µl/well into the plates to start reaction. After reacted for 1.5 hours at room temperature, the plates were fully washed. A β-galactosidase-labeled anti-human apo(a) monoclonal antibody solution was then poured in portions of 100 µm/well into the antibody-bound plates to conduct reaction for 1.5 hours at room temperature. After fully washing the plates, O-nitrophenyl-β-D-galactopyranoside was used as a substrate to perform an enzyme reaction at room temperature. After 1.5 hours, a 0.1 M Na 2 CO 3 solution was added in portions of 100 µl/well to the plates to stop the reaction. Thereafter, the absorbances of the liquid reaction mixtures in the individual plates were measured at a wavelength of 414 nm (reference wavelength: 690 nm). This example is intended to use the nature of the MDA-modified HSA that reacts with the anti-MDA-modified Lp(a) monoclonal antibody coated on the plate, so as to demonstrate that if the substance detected in the assay of the present invention is MDA-modified Lp(a) in the blood, the MDA-modified Lp(a) undergoes competitive inhibition against the MDA-modified HSA added to the reaction system. The results are as illustrated in FIG. 7 and revealed that the addition of HSA does not affect the reaction, but the addition of MDA-modified HSA inhibits the reaction in proportion to the concentration thereof. This indicates that the assay of the present invention can specifically detect the MDA-modified Lp(a) in the blood. Example 8: Assay of MDA-modified Lp(a) added with sodium dodecylbenzenesulfonate as anionic surfactant Antibody-bound plates prepared in the same manner as in Example 6 were used in reaction. The following assaying procedure was followed. Namely, solutions of BSA-HEPES (see Example 6), to which sodium dodecylbenzenesulfonate had been added in concentrations of 0, 1, 5 and 10 mM, respectively, were added in portions of 100 µl/well to the antibody-bound plates. Further, dilute samples of a human serum, which had been obtained by diluting the serum from 1/50 to 1/400 in four steps with BSA-HEPES, were separately poured in portions of 50 µl/well into the antibody-bound plates to start reaction. Thereafter, the procedure of Example 7 was followed to conduct the succeeding steps of the assay. The results are as illustrated in FIG. 8, and revealed that in the reaction system to which no sodium dodecylbenzenesulfonate is added, the MDA-modified Lp(a) in the serum cannot be detected at all, while the MDA-modified Lp(a) in the serum can be determined by adding sodium dodecylbenzenesulfonate to the reaction system. Example 9: Assay of MDA-modified Lp(a) added with cetyltrimethylammonium chloride as cationic surfactant Antibody-bound plates prepared in the same manner as in Example 6 were used in reaction. The following assaying procedure was followed. Namely, solutions of BSA-HEPES (see Example 6), to which cetyltrimethylammonium chloride had been added in concentrations of 0, 5, 10 and 20 mM, respectively, were added in portions of 100 µl/well to the antibody-bound plates. Further, dilute samples of a human serum, which had been obtained by diluting the serum from 1/100 to 1/400 in three steps with BSA-HEPES, were separately poured in portions of 50 µl/well into the antibody-bound plates to start reaction. Thereafter, the procedure of Example 7 was followed to conduct the succeeding steps of the assay. The results are as illustrated in FIG. 9, and revealed that in the reaction system to which no cetyltrimethylammonium chloride is added, the MDA-modified Lp(a) in the serum cannot be detected at all, while the MDA-modified Lp(a) in the serum can be determined by adding cetyltrimethylammonium chloride to the reaction system. Example 10: Assay of MDA-modified Lp(a) added with polyoxyethylene secondary alkyl ether as nonionic surfactant Antibody-bound plates prepared in the same manner as in Example 6 were used in reaction. The following assaying procedure was followed. Namely, solutions of BSA-HEPES (see Example 6), to which a polyoxyethylene secondary alkyl ether had been added in concentrations of 0, 0.05, 0.2 and 0.5%, respectively, were added in portions of 100 µl/well to the antibody-bound plates. Further, dilute samples of a human serum, which had been obtained by diluting the serum from 1/300 to 1/2700 in three steps with BSA-HEPES, were separately poured in portions of 50 µl/well into the antibody-bound plates to start reaction. After reacted for 1.5 hours at room temperature, the plates were fully washed. A β-galactosidase-labeled anti-human apo(a) monoclonal antibody solution was then poured in portions of 100 µm/well into the antibody-bound plates to conduct reaction for 1.5 hours at room temperature. After fully washing the plates, O-nitrophenyl-β-D-galactopyranoside was used as a substrate to perform an enzyme reaction at room temperature. After 30 minutes, a 0.1 M Na 2 CO 3 solution was added in portions of 100 µl/well to the plates to stop the reaction. Thereafter, the absorbances of the liquid reaction mixtures in the individual plates were measured at a wavelength of 414 nm (reference wavelength: 690 nm). The results are as illustrated in FIG. 10, and revealed that in the reaction system to which no polyoxyethylene secondary alkyl ether is added, the MDA-modified Lp(a) in the serum cannot be detected at all, while the MDA-modified Lp(a) in the serum can be determined by adding the polyoxyethylene secondary alkyl ether to the reaction system. Example 11: Sera of 25 persons of health, 13 diabetics and 10 cases of an arteriosclerotic disease were used as specimens to determine the amount of MDA-modified Lp(a) contained in each serum according to the method of Example 7. Namely, antibody-bound plates prepared in the same manner as in Example 6 were used in reaction, and the following assaying procedure was followed. Each of the sera diluted to 1/50 and BSA-HEPES containing 6.75 mM of SDS were added in proportions of 50 µl/well and 100 µl/well, respectively, to the antibody-bound plate. Thereafter, the procedure of Example 7 was followed to conduct the succeeding steps of the assay. The results are as illustrated in FIGs. 11(1)-11(3), and revealed that the MDA-modified Lp(a) exists in plenty in the sera of the cases of diabetes and arteriosclerotic disease compared with the sera of the persons of health.

Claims: An assay of a denatured lipoprotein, which comprises: exposing the denatured site of the denatured lipoprotein contained in a biological sample to the surface of its lipoprotein particle and reacting an antibody, which recognizes the denatured lipoprotein, with the biological sample containing the denatured lipoprotein. The assay according to Claim 1, wherein the exposure of the denatured site of the denatured lipoprotein to the surface of its lipoprotein particle is performed by causing a surfactant to coexist in the reaction system containing the biological sample and the antibody which recognizes the denatured lipoprotein. The assay according to Claim 1 or 2, wherein the antibody which recognizes the denatured lipoprotein is an anti-malondialdehyde-modified lipoprotein antibody. The assay according to Claim 2, wherein the surfactant is an anionic surfactant. The assay according to Claim 4, wherein the anionic surfactant is an alkylsulfate. The assay according to any one of Claims 1-5, wherein the denatured lipoprotein is selected from the group consisting of denatured low density lipoprotein, denatured lipoprotein (a) and denatured high density lipoprotein.

Inventors: KOBAYASHI NORIKO, KONDO AKIRA, MANABE MITSUHISA, NOZAWA MASAYUKI, TODA NAOKO, KOBAYASHI, NORIKO,                             LTD, KONDO, AKIRA,                                  TD., MANABE, MITSUHISA,                             LTD, NOZAWA, MASAYUKI,                              TD., TODA, NAOKO,                                   D., KOBAYASHI, NORIKO, C/O DAIICHI PURE CHEM. CO., LTD, KONDO, AKIRA, C/O DAIICHI PURE CHEMICALS CO., LTD., MANABE, MITSUHISA, C/O DAIICHI PURE CHEM. CO., LTD, NOZAWA, MASAYUKI, C/O DAIICHI PURE CHEM. CO., LTD., TODA, NAOKO, C/O DAIICHI PURE CHEMICALS CO., LTD.
Applicants: DAIICHI PURE CHEMICALS CO LTD, DAIICHI PURE CHEMICALS CO., LTD.
Application year: 1994
Main classification: G01N33
IPC codes: G01N33, C07K16, C07K14